Timothy Lyons, PhD , University of California (Riverside)
Introduction and purpose of project
The stated goal of our research was to develop a geochemical baseline for indentifying ancient lakes based solely on inorganic geochemical and sedimentological information. We focused on modern lakes with widely different chemistry and redox conditions to establish a baseline baseline for fingerprinting possible nonmarine contributions to the geologic record. We expect this approach will be useful for determining depositional conditions of ancient shales, particularly as we go further back to the Proterozoic when ocean chemistry may well have approximated modern freshwater (or low sulfate) lacustrine conditions.
Our previous narrative provided an update of progress within the Great Salt Lake, the freshwater transition in the Black Sea, Mahoney Lake, Lake Champlain and the Saint Lawrence Seaway. Here we provide an update on new and ground-breaking results from Lake Champlain and Mahoney Lake.
The Lake Champlain region (NY and VT) experienced a transition from freshwater to marine to freshwater deposition as a consequence of isostatic adjustments to post-glacial sea level rise. The marine stage known as the Champlain Sea as it developed into the modern freshwater Lake Champlain approximately 9 kyr ago is well studied by previous investigators and well constrained temporally and spatially. The marine-lake transition offers the unique constraint of two well-established endmember environments that include modern marine conditions (with sulfate of known concentration and isotopic compostion) and a large (1100 km2) freshwater lake.
Leveraging on our previous success in illustrating a pronounced change in sulfur inventory across most recent transition from the Champlain Sea to the modern Lake Champlain, we turned our attention to extracting the sulfate ions trapped in the crystalline matrix of carbonate minerals. Carbonate-associated sulfate (CAS) offers proxy evidence for the d34S of ancient seawater, a key piece in our interpretation of both flux dynamics and primary/diagenetic redox conditions within the basin. Previous research into CAS has shown that sulfate can be structurally substituted into authigenic and biogenic carbonates. Following wide ranging efforts to calibrate and validate this proxy, it has been demonstrated that the isotopic composition commonly reflects the ambient waters at the time of carbonate precipitation.
Also yielded from the CAS proxy are sulfur concentrations and related metals. Unique to each water source, we expect these data to reflect the combined waters of the Champlain Sea. XRD results will independently dissect the transition by identifying the mineral composition through the transition while also determining the loss order of carbonate and sulfate within the basin. In this respect, we are in a unique position to comment on the lower limits of the CAS proxy as a basin transitions out of a marine setting. By capturing the transition to modern Lake Champlain we hope to quantify the dynamics of this evolution including the relative strengths of the competing fluxes and how the basin responded to a change in available aqueous species and organic matter input at a time of deglaciation and isostatic rebound.
Mahoney Lake is a shallow (14 m) meromictic lake located in British Columbia, Canada, likely formed 9 Kyr ago following glacial retreat. The alkaline water chemistry has unusually high concentrations of dissolved sulfate in excess of 300 mM and dissolved sulfide levels above 30 mM. The anoxic and sulfidic (euxinic) water column represents extreme reducing conditions beyond traditional end member environments such as the Black Sea. The oxic-anoxic interface supports a dense microbial plate populated by a single species of purple sulfur bacteria (Amoebobacter purpureus) that thrive where the dissolve sulfide gradient intercepts the photic zone. The elevated sulfide concentration coupled with intense microbial activity make this lake an ideal study site to track sulfur redox transformations, particularly the sulfur and oxygen isotopic composition of dissolved sulfate (either interstitial or water column).
We collected water column and sediment samples in July 2008 in collaboration with Dr. Ann Pearson (Harvard) and Dr. David Fike (Washington University in St Louis). Dr. Pearson is an expert in microbial ecology and organic geochemistry. Dr. Fike is an inorganic geochemist with specific interest in studying geochemical gradients at the microbial-scale (sub-micron).
Our isotope dataset from the water column and sedimentary sulfur species (AVS, pyrite, total organic sulfur) provides a fingerprint of biogeochemical signals associated anoxygenic photosynthesis. Downcore sulfate isotope profiles (d34SSO4 = 22.1ä; d18OSO4 = 17.3ä) within oxic and euxinic sediments remain unchanged with depth reflects the vast reservoir of sulfate (400 mM) available for sulfate reduction; however, isotopic enrichments in the water column suggest coupled sulfate reduction and sulfide reoxidation processes at the chemocline. Respective isotopic enrichments in sulfate sulfur and oxygen, by as much as 5ä and 3ä, are observed across the chemocline. Porewater sulfate isotopes are near identical to the maximum isotopic compositions attained above the sediment water interface (d34SSO4 = 28.4ä and d18OSO4 = 20.7ä). Paired pyrite extracts (CRS) also reflect the isotopic composition of water column sulfides suggesting that the sulfur buried (as pyrite) in the euxinic depocenter of the lake record redox transformations driven by microbial activity within the plate situated at the chemocline.
A novel molecular genetic approach was employed in collaboration with Dr. Pearson to describe the entire microbial community of the water column and sediments. Previous research efforts have focused mainly on the plate of purple sulfur bacteria established at the oxic-anoxic interface in the water column. PhyloChip microarrays were used to profile the distribution of 16S rRNA genes extant in the water and sediment. Results demonstrate that microbial diversity increases with depth, with the greatest diversity found within the monimolimnion and the sediments. Taxa affiliated with cycling sulfur intermediates were found within near the oxic-anoxic interface at the chemocline and sulfate reduction was the dominant microbial process at depth.